Flotation processes are widely used in the art to concentrate and separate ore constituents present in naturally-occurring deposits, such as feldspar. A conventional flotation process comprises the steps of crushing and grinding the ore material to reach a suitable degree of liberation, conditioning the ground material under high acidic conditions with a promoter, such as hydrofluoric acid or sodium fluoride, to provide an aqueous suspension having pH of about 2-3, and floating the material to form a feldspar-containing concentrate and a waste product, commonly referred to as “tailings”.
The aqueous suspension is normally conditioned by treating with a so-called “collector”, normally a cationic amine-type surfactant. A foaming agent may be further added. Flotation is carried out by bubbling air or nitrogen through the suspension. Feldspars are then recovered in the froth or foam thus generated, while the other constituents, such as quartz, remain in the tailings.
Hydrofluoric acid (HF) is traditionally used in plant for conditioning feldspars because it acts as a pH regulator and an activating agent. As a result, HF provides the process with excellent selectivity. However, the toxicity of HF and its ecological impacts from the potential fluoride ion contamination of both surface and subsurface water sources are relevant limiting factors for the use of HF in industrial scale processes. In fact, HF requires careful handling procedures and additional steps are needed for disposing of the water used during flotation, thus significantly increasing process costs. Moreover, HF has the disadvantage of creating a corrosive chemical environment in the flotation cells and processing equipment. Therefore, the interest for new processes free from HF is always growing.
Various processes to separate feldspar from quartz have been developed in the art to try to limit or avoid the use of HF; in particular, various non-fluoride reagent systems, comprising mixtures of cationic and anionic collectors, have been proposed in the art. U.S. Pat. No. 3,844,939 describes a froth flotation process wherein hydrofluoric acid is substituted by sulfuric acid as the activating agent, and the collector system comprises a combination of a higher aliphatic amine salt (such as tallow diamine acetate) and a higher aliphatic or aromatic sulfonate; the pH of the flotation pulp is adjusted to 2-3.5. Besides the high reagent cost, this process has the disadvantage of providing an unsatisfactory recovery of feldspar from high feldspar containing ores. Moreover, if the pH is not carefully controlled, sulfuric acid reacts with the higher aliphatic amine to form a precipitate, thus providing feldspar of unsatisfactory quality and requiring additional flotation steps.
An alternative approach is disclosed in U.S. Pat. No. 4,038,179, wherein hydrochloric acid is used instead of sufluric acid, and the collector system comprises a mixture of petroleum sulfonate and N-higher alkyl-alkylenediamine salt.
A. Vidyadhar et al., Journal of Colloid and Interface Science 248, 19-29 (2002), disclose the use of a mixed collector system comprising a cationic alkyl diamine and oleate, at pH of 2. This system shows very low feldspar recovery levels (less than 60% albite recovery, as indicated in FIG. 17, page 28) and low feldspar selectivity. Therefore, when compared to conventional HF flotation processes, the above solutions are still unsatisfactory; it is thus felt the need for a flotation process which does not require the use of hazardous chemicals such as HF, and which at the same time is able to provide improved recovery levels of feldspar, with high selectivity towards feldspar.